[Elecraft] Non-Resonant Antennas and Chokes

[Al Lorona]

>>> the feedline is a wire dangling from the antenna 
>>> that isn't connected to anything on the other end.

That "dangling wire" is actually *two* wires, and the field of one cancels the field of the other for no net radiation or reception -- at least that is the condition we're trying to achieve. That isn't the same as a single wire which by definition would be a common-mode conductor as you correctly say.

Once common-mode current is reduced to a small enough value, then the open-wire line isn't radiating nor receiving. But... you gotta measure the common-mode current to know. I have done so. The instrument used to measure this is quite simple.

>>> a choke that doesn't fry with TX 
>>> power probably isn't doing anything useful.

A choke dissipates power only in it's resistance, not it's reactance, and only due to the common-mode current. If the parameters of the choke are chosen correctly for the frequency band, and if the choke reduces common-mode current to a low enough value, then the power dissipated in the choke can be very low even when you're operating at high power. A choke that burns up at high power is certainly not inevitable nor normal and can be fixed by re-designing the choke. There are a number of ferrite mixes available and they seem to each be optimum for a slightly different part of the HF spectrum.

There's also the twist that permeability is actually a complex quantity (real and imaginary parts) which has a direct bearing on the resistance and reactance of the choke, but I won't go into that at this time.

The posting limit is fast approaching; I'll let you have the last word.

Al  W6LX